(1). Field of the Invention
The present invention relates to an optical master disk exposure apparatus and method, and more particularly to an apparatus and a method for use in producing an optical disk medium having high signal quality.
(2). Description of the Related Art
An optical disk has a plurality of concentric or spiral guide grooves and a plurality of pre-pits. Such data as address data are recorded as pre-pits in a region being sandwiched between two grooves adjacent to each other. For providing an optical disk having high signal quality, it is necessary that the shapes of the guide grooves adjacent to the pre-pits and the shapes of the guide grooves not adjacent to the pre-pits be made uniform in the entire region of the surface of the disk, and also that cross-talk to a track adjacent to the pre-pits be substantially reduced. There are demands for an optical master disk exposure method and apparatus which enables the production of such an optical disk as explained.
Optical disks, with which data can be written, erased and read using a laser beam, have excellent characteristics as high capacity memory devices because of their high recording density. Expanded application thereof is being widely researched and developed in fields of such memory devices as for code data and image data.
With recording density increase of such optical disks, the data formats employed are such formats as an LTF (Logical Track Format) and a zone CAV (Constant Angular Velocity) as ISO formats of 5.25-inch and 3.5-inch optical disks in which the linear recording density is made uniform over the entire disk surface. In these formats, pre-pits are not all oriented in a particular central angular direction in the disk surface.
In an exposure apparatus for an optical master disk in which such data as address data are recorded as pre-pits in a region defined between two concentric or spiral guide grooves, the guide grooves and the pre-pits are exposed by using different laser beams. Such an apparatus is called a two-beam exposure. In another well-known technique, the power of the laser beam used for the guide groove exposure is set to different levels for guide grooves adjacent to pre-pits and those not adjacent to pre-pits. By so doing, the guide grooves adjacent to pre-pits and those not adjacent to pre-pits are made uniform in their shapes, thus improving the signal quality of the groove.
For example, in the arrangement disclosed in Japanese Patent Application Kokai Publication No. Hei 2-73543, the exposure of address bits for writing track numbers and sector numbers with respect to the optical master disk is made by a first laser beam and the exposure of V-shaped guide grooves for tracking is made by a second laser beam which is arranged so as to be spaced from the first laser beam by one half of the track pitch and in parallel with the first laser beam. The exposure of the V-shaped guide grooves is performed such that the power of the exposure laser applied to the portion where there are address bits is reduced from that applied to the portion where there are no address bits. In this way, it is possible to prevent the film thickness from being reduced at the two side portions of the pits.
Also, Japanese Patent Application Kokai Publication No. Sho 63-149846 discloses an arrangement wherein one groove is formed by the exposure of a photoresist film with a plurality of laser beams whose centers deviate from one another and being disposed so as to overlap one over another. In this way, even when the depth of the guide groove is less than the thickness of the photoresist film, the groove can be formed so as to have a nearly rectangular flat bottom.
From the optical master disk which has been exposed in the above way, a stamper used to form optical disk media is produced using steps such as development, electro-casting and polishing.
The optical disks which are obtained in the above way are already at a practical application level in code data file memory fields and, also for still image and movie image data fields. The application of the optical disks is expected to be further expanded and diversified in multimedia fields.
With the increase in the application of these optical disks, recording density improvement or further operation speed increase is expected through, for example, a reduction of the inter-guide groove interval.
When a stamper produced by using an optical master disk produced by the above prior art exposure method is used to produce an optical disk medium having a data format in which pre-pits are not all oriented in a particular central angular direction as in the LTF method and the zone CAV method, the pre-pits and guide grooves adjacent thereto are exposed with laser beams having Gaussian distributions. Therefore, the laser beam for exposing the pre-pits and the laser beam for exposing the guide grooves partly overlap each other. This leads to a problem that a portion of a guide groove that is adjacent to a pre-pit is different in shape from a portion of a guide groove not adjacent to a pre-pit, and also to a problem that the groove signals produced at internal groove surfaces become non-uniform.
A further problem in the prior art apparatus and method for optical master disk exposure is that, since the change in the shape of the guide grooves adjacent to the pre-pits increases the cross-talk which is caused by the leakage of adjacent track pre-pit signals into regions where no pre-pits are originally present, the recorded data signal quality is deteriorated.
FIG. 1 shows an example of shapes of the guide groove adjacent to pre-pits in the optical disk stamper produced by a conventional optical master disk exposure apparatus, and is for use in explaining causes for making the cross-talk large. In the region of the guide groove 12 which is adjacent to the pre-pits 11, the guide groove 12 is expanded toward a track side with the pre-pits 11, as shown in FIG. 1. It is considered that, as a result of the above, the laser beam used during playback passes through the region closer to the track with the pre-pits, thus increasing the cross-talk from the adjacent track.
In a conventional method, as a means to overcome the above explained problem, the power level of the laser irradiated is changed between the guide groove adjacent to pre-pits and the guide groove not adjacent to the pre-pits in an attempt to uniformly form the guide groove adjacent to the pre-pits and those not adjacent to the pre-pits.
FIG. 2 shows exposure pulses at the guide groove and the pre-pits of the conventional optical master disk exposure apparatus, for showing the relationship among an index signal, a pre-pit record signal and a guide groove record signal.
In the conventional exposure method, the laser beam to expose the pre-pits and the laser beam to expose the inside or outside of the guide groove are of one set for making the exposure, so that the irradiation power of the laser beam exposing either the inside or the outside thereof adjacent to the pre-pit is so modulated that, in the region adjacent to the pre-pit, the power is lower than that in the region adjacent to the pre-pit. Thus, at the region where the guide groove is adjacent to the pre-pits, the guide groove can be formed uniformly with respect to the guide groove not adjacent to the pre-pits.
However, the guide groove oppositely located with respect to the guide groove that is exposed simultaneously with the pre-pits, with the pre-pits interposed therebetween, that is, the guide groove not exposed with the pre-pits as one set, is exposed by the laser beam having the same power level as that of the laser beam used when exposing the guide groove at the region where the pre-pits are not present. Thus, at the region where a portion of the beam waist of the laser beam for exposing the pre-pits and a beam waist of the laser beam for exposing the guide groove overlap with each other, and this results in the same effects as those observed in the case where the laser irradiating power was large, thus leading to a problem that the shape of the guide groove at one side which is adjacent to the pre-pits becomes different from that of the guide groove at the region not adjacent to the pre-pits as shown in FIG. 1.